What type of person do I hire to do such a conversion? Can a GC do it or am I better of with some sort of consultant helping the process along?

: ++ What's 'normal'? You'd change when the old steam boiler died. Then it would be worth it over the next decades.++++That sounds like at best a 10+ year payback versus just a new steam boiler.

: : How can you run the water at the same temperature as the steam?: ++ Set the high limit to 225F. Steam or water inside a radiator at 215F provides the same heat output.++++Won't the water be steam above 212F or not because of the system pressure or does it get superheated? Can PEX take such hot water?

: ++ When the new boiler is a water boiler it will be smaller right away as you'd size for the actual heat loss instead of total steam radiation plus 50%.++++ Is the heat loss always less than the steam system size? What about rooms that haven't needed heat with the steam system such as an enclosed porch mudroom? I am concerned these may not get warm enough with hot water.